As is known in the art, current emission control regulations necessitate the use of catalysts in the exhaust systems of automotive vehicles in order to convert carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) produced during engine operation into harmless exhaust gasses. Vehicles equipped with diesel or lean gasoline engines offer the benefits of increased fuel economy. Such vehicles have to be equipped with lean exhaust aftertreatment devices such as, for example, a reductant such as urea, in a urea-based Selective Catalytic Reduction (SCR) catalyst, which is capable of continuously reducing NOx emissions, even in an oxygen rich environment. Urea-based SCR catalysts use gaseous ammonia as the active NOx reducing agent. Typically, an aqueous solution of urea is carried on board of a vehicle, and an injection system is used to supply it into the exhaust gas stream. The heat in the exhaust gas causes the aqueous urea solution to decompose into ammonia and hydro-cyanic acid (HNCO). These decomposition products enter the SCR where the gas phase ammonia is adsorbed and the cyanic acid is further decomposed on the SCR to gas phase ammonia. The adsorbed ammonia then takes part in the reduction of gas phase NOx. Selective Catalyst Reduction (SCR) system is proved to be reliable device for NOx reduction; however, the disadvantage of this system is that the overall performance is dependent of urea injection system. Water-urea solution is injected into exhaust stream, as shown in FIG. 1, and decomposes into NH3 and CO2 when heated up by exhaust gas. Certain length of pipe is needed to achieve an ideal mixing of NH3 in front of substrate; however it is usually not the case with stringent package constrain. In general, a flow mixer or atomizer is implemented right after the urea injector. It is very difficult to evaluate the performance of those flow-mixing devices experimentally.
The basic chemical reactions inside NOx reduction process by using urea solution are well known. Urea solution will atomize and dissolve as ammonia and carbon dioxide when mixes with exhaust gas of certain temperature as described by following equations:(NH2)2CO→HNCO+NH3 NHCO+H2O→NH3+CO2 
Then gaseous ammonia reacts with NOx to produce Nitrogen and water as described as following:4NH3+4NO+O2→4N2+6H2O8NH3+6NO2→7N2+12H2O4NH3+2NO2+O2→3N2+6H2O
In the past, many flow mixers were developed for above purpose. Almost all the design attentions were focused on changing flow pattern by generating turbulent and swirling flow at the downstream of liquid spray.
The inventor has recognized that in order to achieve a significant improvement of flow mixing performance, besides two intrinsic flow mechanisms: turbulence and bulk rotation, another most important flow mechanism which can improve fluid atomization, evaporation and flow mixing greatly is direct flow impingement.
In accordance with the invention, a selective catalytic reduction (SCR) injection system is provided for mixing reductant with exhaust gasses. The system includes: a plate disposed between walls of an entrance portion of an exhaust pipe, such plate separating such entrance section of the exhaust pipe from an egress section of the exhaust pipe; and a reductant-introducing conduit. The exhaust gasses in the entrance section of the exhaust pipe pass through apertures in a wall of the reductant-introducing conduit. The conduit has an outlet disposed in the egress section of the exhaust pipe.
In one embodiment, a selective catalytic reduction (SCR) injection system is provided for mixing reductant with exhaust gasses. The system includes a plate disposed between walls of an exhaust pipe separating an entrance section of the exhaust pipe from an egress section of the exhaust pipe. The plate intercepts exhaust gasses entering the entrance section of the exhaust pipe and directs such exhaust gasses through apertures in a wall of a reductant-introducing conduit. The conduit has an outlet disposed in the egress section of the exhaust pipe.
With such an arrangement the system: increases flow impingement: help for droplet atomization and evaporation; increases turbulent intensity; help for evaporation and flow mixing; and increase bulk rotation help flow mixing.
In one embodiment, the dimensions of the apertures in the wall and dimensions in the outlet of the conduit are selected to increase the velocity of the exhaust gases leaving the outlet of the conduit into the egress section relative to the velocity of the exhaust gases in the entrance section of the exhaust pipe.
In one embodiment, the conduit is a perforated conduit and the plate is a “Z” shaped plate, such plate having a hole therein for receiving the conduit outlet.
In one embodiment, the outlet of the conduit is at lower portion thereof and the upper portion thereof is adapted to receive reductant.
In one embodiment, the plate has perforations passing portions of the exhaust gases in the entrance section of the exhaust pipe into the egress section of the exhaust pipe to reduce pressure in the entrance section of the exhaust pipe.
In one embodiment, the plate has perforations passing portions of the exhaust gases in the entrance section of the exhaust pipe into the egress section of the exhaust pipe to assist in creating a second impingement between exhaust gas and flows existing from injection conduit outlet and wherein first impingement occurring between exhaust gas entering the conduit through the apertures and reluctant in the conduit.
In one embodiment, the plate has perforations in an upper portion of the plate between the conduit and an upper wall portion of the exhaust pipe to assist in creating a second impingement between exhaust gas and flows existing from injection conduit outlet to reduce pressure in the entrance section of the exhaust pipe.
In one embodiment, the plate has perforations in a lower portion of the plate between the conduit and a lower wall portion of the exhaust pipe to assist in creating a second impingement between exhaust gas and flows existing from injection conduit outlet and to reduce pressure in the entrance section of the exhaust pipe.
In one embodiment, the plate has perforations in a lower portion of the plate between the conduit and a lower wall portion of the exhaust pipe to assist in creating a second impingement between exhaust gas and flows existing from injection conduit outlet and also in an upper portion of the plate between the conduit and an upper wall portion of the exhaust pipe to assist in creating a second impingement between exhaust gas and flows existing from injection conduit outlet and to reduce pressure in the entrance section of the exhaust pipe.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
Like reference symbols in the various drawings indicate like elements.